Simultaneous broadcast, or simulcast transmission systems, typically have a plurality of remote transmitters that simultaneously broadcast identical audio or information signals at substantially the same carrier frequency. By using a number of geographically diverse transmitters, a simulcast system provides maximum signal coverage to topographically irregular geographical areas, such as mountainous regions, where unobstructed terrestrial radio coverage is not possible.
Simulcast two-way radio systems are used when a large geographic area is to served and there are few frequencies available. Simulcast systems blanket large areas since it is unknown where mobile communication units are located in the system. For example, in a ten site simulcast system, all the units receiving one call are typically located near just a few of the sites in the system. However, simulcast transmissions emit from all ten transmitter sites since today's simulcast systems cannot determine the actual sites required. Users experience degraded audio quality in areas of signal overlap coverage. Multiple site overlap regions frequently occur.
Data communications systems having multiple transmitter sites exist which remove distortion. The geographical coverage area is divided into non-overlapping zones. Each particular zone has a transmitter designated to it, although multiple zones may be assigned to the same transmitter, and when a call originates from that zone, only that transmitter may be keyed on that frequency. A frequency may be reused within the system only if the transmissions do not take place within the same coverage area of the sites transmitting on that frequency. The same message is not transmitted on that frequency from more than one site using the frequency. Data communications are typically one user calling another user, i.e. only one receiving entity. To complete a transmission usually requires only one base site transmitting the message. The methods used for a data communications system call are not readily applicable to a simulcast voice system, especially where the initiating communication unit desires a group call, when multiple users receive the same call simultaneously on the same frequency.
Accordingly, a mechanism to allow simulcast voice communication in group calls while reducing distortion is required.